We previously reported that the ␤ 1 -adrenergic receptor (␤ 1 AR) associates with PSD-95 through a PDZ domain-mediated interaction, by which PSD-95 modulates ␤ 1 AR function and facilitates the physical association of ␤ 1 AR with other synaptic proteins such as N-methyl-Daspartate receptors. Here we demonstrate that ␤ 1 AR association with PSD-95 is regulated by G protein-coupled receptor kinase 5 (GRK5). When ␤ 1 AR and PSD-95 were coexpressed with either GRK2 or GRK5 in COS-7 cells, GRK5

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... dramatically decreased the association of ␤ 1 AR with PSD-95, although GRK2 and GRK5 both could be co-immunoprecipitated with ␤ 1 AR and both could enhance receptor phosphorylation in vivo. Increasing expression of GRK5 in the cells led to further decreased ␤ 1 AR association with PSD-95. Stimulation with the ␤ 1 AR agonist isoproterenol further decreased PSD-95 binding to ␤ 1 AR. In addition, GRK5 protein kinase activity was required for this regulatory effect since a kinase-inactive GRK5 mutant had no effect on PSD-95 binding to ␤ 1 AR. Moreover, the regulatory effect of GRK5 on ␤ 1 AR association with PSD-95 was observed only when GRK5 was expressed together with the receptor, but not when GRK5 was coexpressed with PSD-95. Thus, we propose that GRK5 regulates ␤ 1 AR association with PSD-95 through phosphorylation of ␤ 1 AR. Regulation of protein association through receptor phosphorylation may be a general mechanism used by G protein-coupled receptors that associate via PDZ domain-mediated protein/protein interactions.